The present invention relates generally to chucks used to hold flat workpieces and specifically to chucks which hold workpieces such as semiconductor wafers and control the temperature of the workpieces.
In the semiconductor integrated circuit industry, the cost of individual integrated circuit chip die is continuing to decrease in comparison to IC package costs. Consequently, it is becoming more important to perform many IC test and evaluation steps while the die are still in the wafer, rather than after the relatively expensive packaging steps have been performed.
Increasingly, in IC processing, semiconductor wafers are subjected to a series of test and evaluation steps. For each step, the wafer is held in a stationary position at a process station where the process is performed. For many processes, it is important that the wafer be held extremely flat. For example, circuit testing is typically performed over a wide temperature range to temperature screen the ICs before assembly into a package. The wafer is typically held on a vacuum platform of a host test machine such as a probing station which electrically tests the circuits on the wafer. The prober includes a group of electrical probes which, in conjunction with a tester, apply predetermined electrical excitations to various predetermined portions of the circuits on the wafer and sense the circuits"" responses to the excitations. To ensure that proper electrical contacts are made and to ensure that the mechanical load applied by the probes to the wafer is known and uniform, it is important to keep the wafer extremely flat.
In a typical prober system, the wafer is mounted on the top surface of a wafer chuck, which is held at its bottom surface to a support structure of the prober. A vacuum system is typically connected to the chuck. A series of channels or void regions in communication with the top surface of the chuck conduct the vacuum to the wafer to hold it in place on the top surface of the chuck. The prober support structure for the chuck is then used to locate the wafer under the probes as required to perform the electrical testing on the wafer circuits.
To allow for temperature screening of the wafer circuits, the chuck can also include a heater for heating the wafer to a desired temperature and a heat sink for cooling the wafer as needed. The prober system in conjunction with the chuck can then be used to analyze performance of the wafer circuits at various temperatures within a predetermined temperature range.
Conventional wafer chucks are formed from multiple components fastened together. For example, a typical chuck can include a lower plate or support for mounting to the prober, a heat sink over the lower plate, a heater over the heat sink and an upper plate or support assembly on which the wafer can be held, the upper plate including the vacuum channels used to conduct the vacuum to the top surface. In conventional chucks, all of these layers are typically held together by bolts, rivets, etc., or other rigid, inflexible mechanical fastening means. Furthermore, the chuck is typically held to the base of the host machine by similar rigid means.
These conventional means for holding the chuck together and holding the chuck to the base introduce mechanical stresses into the chuck structure. When the chuck is subjected to variations in temperature, these stresses tend to cause the chuck to deform, resulting in a loss of flatness of the wafer. The non-flat upper surface of the wafer can introduce inaccuracies into the circuit performance measurements performed by the prober.
The deformation in the chuck is typically caused by different chuck layers having different thermal expansion coefficients, such that, over temperature, different layers will experience different thermal expansion forces. Because the chuck layers are held together rigidly, the difference in forces causes the chuck to warp. As the chuck deforms, expansion forces build-up in the chuck. In most chucks, the clamping forces holding the layers together are sufficient to resist relative radial movement between the layers, and the warp increases. In some chucks, the clamping forces are such that, periodically, they are overcome by the expansion forces, and layers move rapidly in a jerking motion relative to each other to relieve the built-up stresses. This rapid xe2x80x9cpoppingxe2x80x9d motion is highly unpredictable and can introduce substantial wafer shape and/or location errors. Also, because the clamping forces are so high in these systems, the chuck layers are not relieved all the way back to a zero-expansion condition. So, in general, there is always some undetermined amount of deformation in the chuck over temperature.
It will be appreciated that these effects caused by the conventional mechanically constrained chuck assembly are magnified for larger diameter chucks. That is, the stresses introduced in clamping or bolting together a large diameter chuck are greater than those introduced in assembling a small diameter chuck. Larger chucks therefore tend to deform more over temperature than do smaller chucks. Therefore, using conventional wafer chuck techniques, it is becoming increasingly more difficult to hold wafers flat over temperature as wafer diameters continue to increase.
Conventional wafer chucks used for temperature cycling are typically mounted on the prober support structure in a manner which provides for good thermal conduction between the chuck and the prober support structure. In these systems, large amounts of energy dedicated to temperature cycling of the wafer can be lost in the form of heat flow between the prober and the chuck. Also, temperature variations in the prober support structure can cause spatial shifts in the wafer which can cause inaccuracies in the prober circuit testing.
It is an object of the invention to provide a workpiece chuck in which the foregoing disadvantages of prior devices are substantially eliminated.
It is a more specific object of the invention to provide a workpiece chuck for supporting a workpiece which maintains the workpiece substantially flat over a wide range of temperature variations.
It is another object of the invention to provide a workpiece chuck which is held together in a stiff but mechanically non-constrained fashion such that thermal and mechanical stresses in the chuck are reduced.
It is another object of the invention to provide a workpiece chuck which is held to the support structure of a host machine such as a wafer prober machine by a stiff but mechanically non-constrained means, such as vacuum or springs.
It is still another object of the invention to provide a workpiece chuck on which large-diameter semiconductor wafers can be supported and maintained flat during electrical probe testing over a wide range of temperatures.
It is yet another object of the invention to provide a workpiece chuck for supporting a semiconductor wafer, the workpiece chuck being mountable on a base and including means for heating and cooling the semiconductor wafer while maintaining the base at or near an ambient temperature.
It is yet another object of the invention to provide a workpiece chuck for supporting a semiconductor wafer and mountable on a base and including means for heating and cooling the semiconductor wafer, the workpiece chuck providing thermal isolation between the workpiece chuck and the base such that the workpiece chuck exhibits improved energy efficiency.
These and other objects of the invention are realized by a chuck apparatus and method for holding a workpiece in accordance with the invention. The chuck of the invention includes an upper support or assembly on which the workpiece or wafer can be mounted and a lower support or assembly by which the chuck can be mounted to a base such as the support structure of a host machine such as a circuit prober. In general, the upper support is characterized by a first temperature and the lower support is characterized by a second temperature. The chuck also includes non-constraining attachment means which holds the upper and lower supports together and holds the lower support and the base together while allowing substantially continuous relative movement between layers of the chuck caused by thermal expansion forces due to differential temperature effects between the upper support, lower support and the base. By using non-constraining attachment means, such as vacuum or springs or spring washers such as belleville washers which are not clamped with sufficient force to completely constrain the chuck layers against radial movement relative to each other, the mechanical stresses found in the rigidly assembled chucks of prior systems are eliminated. The relative movement between layers is substantially continuous in that the rapid jerking or popping motion of prior systems is eliminated by using the non-constraining attachment means.
In one embodiment, the upper assembly can include a substrate made of an insulating material. In one particular embodiment, the insulating material is a ceramic. Where vacuum wafer attachment is used, the substrate can be formed with a vacuum distribution pattern on its top surface for holding the wafer in place. The substrate can be provided with one or more vacuum ports for applying the vacuum to the upper assembly and can include inner channels or void regions connecting the vacuum ports with the pattern on the top surface, or it can be provided with holes through the substrate and metallic surfaces, if any, above and below the substrate to vacuum ports in a lower support or assembly.
The vacuum distribution pattern on the top surface can be a xe2x80x9cwafflexe2x80x9d pattern which includes a rectangular array of raised rectangular regions separated by narrow channels along the surface through which the vacuum is distributed to hold down the wafer. In this configuration, the bottom surface of the wafer rests on the top surfaces of the raised rectangular regions.
The vacuum distribution pattern on the top surface can be formed on the top surface by one of several processes. In one approach, the pattern of channels or xe2x80x9cstreetsxe2x80x9d is ground into the ceramic substrate and may then be coated with a layer of metal if electrical contact to the back side of the wafer is desired. In another embodiment, a uniform layer of metal is deposited onto the top surface of the substrate, and then a pattern of channels is etched into the metal, leaving a pattern of raised rectangular metallic pads. In another embodiment, the raised regions are formed by depositing the array of rectangular metallic pads onto the ceramic substrate, leaving the channels between the pads. To provide electrical conduction between the chuck and the wafer, a thin layer of metal can be added on top of the patterned vacuum distribution layer. Any of the metallic layers can be deposited by a silk screening process, or other process such as plating, sputtering, brazing, etc., or a combination thereof. During circuit testing, to improve the sensitivity of a measurement, it is sometimes desirable to reduce electrical current leakage between the wafer under test and ground. To that end, the substrate in the chuck of the invention can include a guard layer contacting its bottom surface. The guard layer is a layer of metal contacting the bottom surface of the substrate and connected to a terminal to allow for external electrical access. To reduce leakage or capacitance effects in the substrate, a signal approximately identical to the excitation signal being applied to the circuit under test is applied to the guard layer. By thus maintaining the upper and lower surfaces of the substrate at the same potential, leakage currents through the substrate are substantially reduced or eliminated. The guard layer includes an insulating surface below it which permits a signal approximately identical to the excitation signal being applied to the circuit under test to be applied to the guard layer.
Where the upper and lower assemblies are held together by vacuum, the bottom surface of the upper assembly can be formed with another vacuum distribution pattern which may be produced by any of the means by which vacuum patterns can be formed on the top surface of the substrate. The pattern can define plural concentric raised portions with concentric annular vacuum distribution regions between them. The guard layer can be held together with the upper assembly by this lower vacuum distribution pattern in the upper assembly or by a vacuum pattern in the guard layer itself.
The lower assembly can include a heater and a heat sink for heating and cooling the wafer. In one embodiment, the heat sink is located above the heater and, hence, closer to the wafer to provide more efficient heat flow into the heat sink during cooling. The heater can be attached to the bottom surface of the heat sink by one of many techniques, one of which is to directly vulcanize it to the heat sink, another of which involves bonding the heater to the heat sink using epoxy. The heat sink can include tubing through which fluid flows. The tubing can be formed as a spiral intake with a reverse spiral outlet, with intake tubing adjacent to outlet tubing to provide efficient and uniform removal of heat from the heat sink. In one embodiment, fluid can also flow through the bottom of the lower assembly to maintain an ambient temperature barrier between the chuck and the base to prevent heat flow between the chuck and the base.
The upper and lower assemblies can be aligned with each other by one or more alignment pins. In one embodiment, the alignment pins are pressed into the lower assembly and protrude through the top surface of the lower assembly. When the upper and lower assemblies are brought together, the alignment pins mate with alignment holes in the bottom surface of the upper assembly. In one embodiment, one of the alignment holes, which can be located at the center of the chuck, is round and is sized to provide a slip fit with its associated alignment pin. Another hole can be elongated to provide a slip fit with a pin in one direction and to allow motion of the pin in the orthogonal radial direction. This configuration allows for relative expansion and contraction of parts while preventing relative rotation.
The present invention provides thermal isolation between the chuck and the base while also providing adequate mechanical support for the mechanical load on the wafer, such as that due to forces exerted by the probes or probe array of the prober. The lower assembly includes a lower support plate to which the heat sink can be mounted. The heat sink can rest on a plurality of thermally insulating elements located between the bottom of the heat sink and the top of the lower support plate. The elements can be in the shape of posts, rods, cylinders or spherical balls in any spatial orientation including upright or lying down, and can be made of a thermally insulating material such as glass, ceramic, etc. The elements provide thermal isolation and mechanical support within the lower assembly of the chuck. The number of elements and their locations can be selected based on a desired chuck stiffness. For reasonable stiffness of larger diameter chucks, more than three elements, which are sufficient to define a plane, can be used. For this reason, a plurality of elements which have very close-toleranced heights can be used.
The bottom of the heat sink can also be equipped with a vacuum seal such that the chuck can be vacuum mounted to the base. A ring can be mounted to the bottom of the heat sink. The ring can include the required seal, e.g., o-rings, to seal the bottom of the chuck to the base. The ring can also include openings through which additional rods can pass to support the chuck on the base while also providing thermal isolation.
A temperature control system that can be used to control temperature of the chuck and workpiece in accordance with the present invention is described in a copending U.S. patent application entitled xe2x80x9cTemperature Control System for a Workpiece Chuck,xe2x80x9d filed on even date herewith and assigned to the same assignee as the present application. The electrical control system used in connection with the chuck of the present invention is described in a copending U.S. patent application entitled xe2x80x9cPower and Control System for a Workpiece Chuck,xe2x80x9d filed on even date herewith and assigned to the same assignee as the present application. Both of those copending patent applications are incorporated herein in their entirety by reference.